The present invention relates to apparatus and methods for fusion-welding of creviced or defective, e.g., cracked, components having contaminants, e.g., liquid, oxides and the like, trapped in the components and particularly relates to underwater fusion-welding of creviced or cracked components to preclude expulsion conditions leading to porosity and/or blowholes through the weld material.
Intergranular stress corrosion cracks, fatigue cracks, unfused weld material, base metal laminations, joint crevices and other defects which are open to the surface of components submerged in a liquid, e.g., water, trap the liquid within their volume. For example, if the defect has been exposed to a high-temperature liquid, e.g., water environment during service, such as in a nuclear reactor, there is necessarily also an oxide on the creviced or defective surfaces. With the oxide being present, heat from fusion-welding processes typically reduces the oxide to metal and free oxygen, causing increased, unacceptable gas expansion and disruption of the weld pool. Even if oxides are not present, when the heat of a welding process is applied to the defective component, the liquid in the crevice or crack turns to a vapor and causes the weld pool to be displaced by the expanding gas. It will be appreciated that the expulsion condition, e.g., out-gassing, leads to porosity and/or blowholes, preventing crack-like defects or crevices in underwater components from being successfully sealed, despite repeated attempts at welding them closed. These problems also occur in crevices resulting from fitting of components together for joining by welding when at least one of the mated component surfaces has been in high-temperature water service.
To applicants' knowledge, there is no known conventional method for remotely cleaning and sealing water-containing or oxide-containing cracks or crevices so that conventional fusion-welding can be performed thereafter without substantial risk of formation of gas blowholes or internal porosity. Current practices, when confronting this problem, are to remove and replace the entire defective component in a dry environment. An alternative solution is to install mechanically-fastened clamps and bolting to reinforce the cracked area/joint. Such joints, however, introduce undesirable crevices, e.g., leading to corrosion, of their own and consume additional space which, in certain instances, is at a premium. Another alternative is to fully remove the crack or crevice with an underwater excavation process to remove the contamination. This, however, requires additional expense and is fraught with technical difficulty, particularly because it is difficult to determine when the entirety of the cracking has in fact been removed. A still further alternative is to weld directly over the open wet and contaminated defect and to suffer the low-quality caused by the resulting blowholes and/or porosity. In this case, additional weld reinforcement may be required to compensate for reduced weld quality. A still further alternative is to drain the liquid from around the component requiring repair. In many cases, however, such as in a nuclear reactor vessel, this leads to prohibitive radiation levels for welding operators and equipment. Accordingly, there is a need for a welding process for rehabilitating cracked components in a manner to reduce or eliminate expulsion, e.g., out-gassing associated with welding substrates containing wet and/or contaminated cracks or crevices.